1. Field of the Invention
This invention relates to a method and apparatus for heating a plastic extruding die, and more particularly to a method and apparatus for heating a plastic extruding die capable of controlling the area from the base to tip of nozzles at an appropriate temperature. This invention also relates to a plastic extruding die capable of lengthening the life of the coil for heating the periphery of the nozzles.
2. Description of the Related Art
A plastic extruding die has been proposed in which an inductive coil is provided to surround a large number of nozzles attached to a die used for an extruder and kneader and a high frequency current is passed through the inductive coil thereby to induce an eddy current around the nozzles so that the periphery of the nozzles is directly heated by Joule heat (for example, JP-A-2003-326518).
In the related plastic extruding die, a larger part of the nozzles was heated equally using the inductive extending to exceed half of the nozzle length.
However, the tip of the nozzles is likely to get cool whereas the base of the nozzles is difficult to get cool. This presented the following problem. If the tip of the nozzles is heated at an appropriate temperature, the temperature of the base of the nozzles becomes excessively high. If the base of the nozzles is heated at the appropriate temperature, the temperature of the tip of the nozzles becomes excessively low.
Further, a method for heating a die of an extruder has been proposed in which a coil is provided in the vicinity of a large number of nozzles attached to a die used for an extruder and kneader and a high frequency current is passed through the coil thereby to induce an eddy current around the nozzles so that the periphery of the nozzles is directly heated by Joule heat (for example, Japanese Patent No. 2,969,437).
Further, a method for heating a plastic extruding die has been proposed in which a coil made of a wound wire having a diameter of 1.5 to 3.0 mm φ coated with alumite or ceramic, or flat wire of 2.0×5.0 mm and having a heating resistant temperature of 480° C. to 550° C. is used and a temperature sensor is arranged at the center or in the vicinity of the coil and at least one of the current value and frequency is controlled so that the coil temperature does not exceed the heating resistance temperature (for example, JP-A-2003-62888).
Further, another plastic extruding die has been proposed in which a cool air is passed through the space provided on the outer periphery of a coil to cool the coil (JP-A-2003-326518).
The above method for heating a die of an extruder has presented a problem that the coil itself generates heat by the current flowing through the coil and also is heated by the Joule heat generated in the die so that the coil temperature rises, thus shortening the life of the coil.
The above method for heating a plastic extruding die has presented a problem that the die cannot be heated to a sufficient high temperature because of the limitation by the heating resistant temperature of the coil and if the die is maintained at the high temperature, the coil also becomes the high temperature, thus promoting the deterioration by time passage.
The above plastic extruding die has presented a problem that the contact area of the cold air and coil cannot be increased so that the coil cannot be sufficiently cooled.